Human papilloma virus (HPV) 16 is the major type of HPV which, in association with cofactors, can lead to cervical cancer (49). Studies on HPV have been hampered by the inability to propagate the virus in culture, the lack of animal models and the paucity of virions in clinical lesions. This has led to the development of alternative approaches of antigen production for immunological studies. The conformational dependency of neutralizing epitopes, as observed in experimental animal papillomavirus systems (8, 22) suggests that properly assembled HPV particles are critical for the induction and detection of clinically relevant immune reactivity.
The HPV capsids are formed by 72 pentameric capsomers of L1 proteins arranged on a T7 icosahedral lattice (15). Recently, a number of investigators have demonstrated the production of HPV capsids, i.e. virus like particles (VLP), by utilizing baculovirus, vaccinia virus or yeast expression systems (15,22,45,48,61). The potential of VLPs as subunit vaccines has been demonstrated using the cottontail rabbit papillomavirus (CRPV) (4), the canine oral papillomavirus (COPV) (57), and the HPV11 models (45).
HPV16 infects through the genital mucosa, where benign proliferative lesions are confined. Protection against infection with such a pathogen could be provided by specific (anti-VLP) secretory immunoglobulin A (sIgA) or immunoglobulins G (IgG) in genital secretions. By analogy with existing animal models. HPV16 VLPs-specific antibodies in cervical secretions might help to prevent sexually transmitted infection by HPV16 in women. However, this cannot be formally proven in the absence of an experimental model for genital PV infection and other scenarios requiring cell-mediated immunity cannot be excluded.
Moreover, the mechanism underlying HPV infection is unclear. HPV may directly infect the basal cells of the stratified cervical epithelium at the occurrence of breaches. Alternatively, HPV infection could also occur either directly through Langerhans cells in intact epithelia or indirectly from an HPV-producing keratinocyte, and thus neutralizing antibodies will not be functional as shown for other viruses. This further adds to the difficulty in providing vaccines effective against HPV infection.
Immunosuppressed individuals are more prone to develop cervical carcinoma as compared to immuncocompetent individuals, suggesting the possibility of using immunotherapy. Therapeutic vaccines (87) aimed to the treatment of established HPV infection or HPV associated premalignant and malignant lesions have been investigated during the last ten years (59). Evidence for HPV-antigen-directed immunotherapy against cervical cancer comes from the observations that experimental (13),(34),(83) and natural (82) PV-associated tumours can be controlled by immunization with E6 and E7 preparations. These studies suggested that CTL might be the most effective immunological effector mechanisms. E6 and E7 preparations consisted in either peptides (13), bacterially prepared fusion proteins (82), eukaryotic transfected cells (83) or recombinant vaccinia viruses (34).
Recently, chimeric VLPs carrying the 17kD E7 protein as a fusion with L2 have been shown to induce rejection of syngeneic tumour cells (84) engineered to express L1 and/or E7 ORF (i.e. C3 cells (13) and TC1 cells (85)). This data demonstrates the possibility of providing prophylactic and therapeutic effects in the same vaccine preparation. Salmonella that are attenuated, yet invasive, have been proposed for the delivery of heterologous antigens to the mucosal and systemic immune systems (10). The antigen is delivered by the live Salmonella to mucosal inductive sites, where after priming, antigen-specific B and T cells migrate from the site of induction and mature into effector cells. The migrating IgA-expressing B cells home to different mucosal sites, including the genital tract where they differentiate into IgA secreting plasma cells (32). Thus, oral or nasal immunization can provide protective antibodies in genital secretions Recently, we and others have shown that mucosal immunization with recombinant Salmonella can elicit antibody responses in the genital mucosa of mice and humans (18,37,56).